1. Field of the Invention
The present invention generally relates to methods and apparatus for producing molded parts having isotropic or nearly isotropic properties when the starting material has anisotropic properties and, more particularly, to injection molding processes and apparatus for forming polymeric items and/or products having superior mechanical, electrical, thermal, and chemical properties. Such items include but are not limited to substrates, printed circuit boards, printed wiring boards, connectors, interposers, and the like.
2. Description of the Prior Art
The problems associated with anisotropic materials are well known by manufacturers and users of polymeric materials. Attempts to overcome anisotrophy include filling and/or loading the materials with various fillers thereby restricting shrinkage or directional flow of the polymer as it is filling the mold cavity or being extruded. Successful use of these techniques has been limited and, in some cases, the techniques have actually aggravated the problems associated with anisotrophy.
Anisotrophy can and does affect numerous properties of a material including mechanical, thermal, electrical, and chemical as well as manufacturing or fabricating the material into a useful product. In the manufacture of an item such as a substrate used to attach semiconductors, circuits, via-connectors, and the like, it is desirable to have properties the same in all directions if possible; that is, the substrate should be isotropic in nature. Many isotropic materials, however, possess poor physical properties that do not meet electrical substrate/board requirements.
Important properties to insure success of a molded polymeric item such as a substrate for electrical components include (1) matched (within allowable tolerances) coefficient of linear expansion to the materials and components being used, (2) high thermal conductivities, (3) high continuous use temperature, (4) non-flammable, (5) high chemical resistance, (6) low water absorption, (7) non-corrosive, and (8) processability. Additionally, it is preferable that the materials be low cost and easy to manufacture into required geometries, be capable of surface mounting and vapor phase soldering technologies, be circuitized with various metals and by various techniques, and have long term mechanical stability.
A typical material having desirable properties for electrical substrates is a group of materials called liquid crystal polymers. However, in current injection molding processes, parts produced using these materials exhibit a uniaxial, anisotrophic orientation. Such parts exhibit properties which are highly dependent on orientation.
Recently, molding techniques have been developed which allow the production of parts having defined layers of directionally oriented materials. In particular, U.S. Pat. No. 4,994,220 to Gutjahr et al. discloses a process for injection molding parts from plasticized liquid crystal polymer materials wherein a molten flow of the materials are injected into a mold cavity through at least two different gates positioned at different locations around the cavity at different heights. Gutjahr et al. specifically points out that there are advantages in having fibers in adjacent layers oriented 90.degree. apart. That is, when layers of liquid crystal polymers are staggered at 90.degree., the end product is more isotropic since there will be criss-crossing lines of anisotrophy. The method of Gutjahr et al. does not employ movable gates to produce the oppositely oriented layers in the liquid crystal polymer parts and does not consider the effects of filling pressures and drops on the shinkage in multiple dierections. Other examples of newer molding techniques are found in U.S. Pat. No. 4,925,161 to Allan et al. which shows a process for molding directionally-orientable materials using shear force and in Kirkland et al., "New molding methods increase design freedom", Plastics World, pp. 37-42 (Feb., 1991) which discusses the "live-feed injection molding" described in U.S. Pat. No. 4,925,161 to Allan et al. as well as a push-pull injection molding technique.